Electromagnetic trackers find use in a wide range of applications, ranging from video game systems and computer graphics to medical technologies and military uses. The main purpose of a tracker, in general, is to continuously determine the position of a moving object in real-time. Determining the position may include determining the physical location of the object, and the orientation of the object with respect to a coordinate system. For example, the continuously determined position may be used to control an object, like a medical probe, or to estimate the line of sight to a target.
In particular, electromagnetic trackers function by generating an electromagnetic field in the physical space in which an object is to be tracked. This physical space is referred to as a motion box in the art. The electromagnetic field is generated by one or more transmitters located on the electromagnetic tracker. A sensor is generally attached to the tracked object, which can determine a characteristic property of the electromagnetic field as a function of the physical location of the tracked object. It is common practice to determine a characteristic property of the electromagnetic field which involves the principle of electromagnetic induction. The measure of the characteristic property can be used to determine the position of the tracked object.
The sensor, attached to the tracked object, commonly measures the magnetic field strength at a single point in the motion box of the electromagnetic tracker, either as a vector or as the components of a vector. This single point is by and large related to the geometric center of the sensor. This measure is generally to a high degree of precision and is achieved by using sensors with a high degree of symmetry. For example, if the sensor is coil based, the coils should be concentric to a high degree of precision. Such sensors generally require a sophisticated calibration process, which is normally executed using a specially designed calibration setup. Over time, with general use and wear-and-tear, the geometric properties of a sensor change, and the calibration parameters of the sensor drift from their original values. This change in values necessitates a periodic replacement of the sensor along with a recalibration of the sensor, a process which is both time consuming and not cost effective. Furthermore, when coil based sensors are used in the presence of a non-uniform electromagnetic field, the finite time required to measure the magnetic field strength causes a distortion in that measurement. The amount of distortion increases as the velocity of the sensors increases. If the translational and rotational velocity of the sensors is faster than velocities typical of hand movements, then electromagnetic trackers tend to exhibit a significant decrease in tracking accuracy.
The magnetic field strength, measured by the sensor, is related to the position of the tracked object by a mathematical model describing the electromagnetic field generated by the electromagnetic tracker. The electromagnetic field generated by the electromagnetic tracker results from electric currents flowing in the transmitter, as well as from induced electric currents arising in metallic objects, which are located in the vicinity of the motion box of the electromagnetic tracker. The electromagnetic field generated is therefore dependent on the particular environment in which the electromagnetic tracker is used in. Any change in the position of metallic objects, in the vicinity of the motion box, requires updating the mathematical relations found between magnetic field strength measurements of the sensor and the position of the sensor. It is common practice to account for induced electric currents, which contribute to the electromagnetic field, in the mathematical model, by mapping the magnetic field strength for each position in the motion box, in a particular environment. The mathematical model parameters are then estimated to best fit the position and magnetic field strength measurements of the sensor to the mapping. The parameters are stored in the memory of the electromagnetic tracker and are used to estimate the position of the sensor in real-time.
In order for the electromagnetic tracker to work in multiple environments, multiple mappings of the electromagnetic field need to be executed in a variety of environments. Furthermore, metallic objects in the vicinity of the motion box of the electromagnetic tracker may change their position, or new metallic objects may be present in the vicinity of the motion box. Both of these situations necessitate a remapping of the motion box. These mappings can be very time consuming. Taken in conjunction with the recalibration needs of the sensor, the maintenance of an electromagnetic tracker can be cumbersome. In general, the analog hardware used in electromagnetic trackers needs to be of high precision and stability, which enables the analog hardware to be replaced when needed without changing the calibration parameters of the sensor or the estimated parameters of the mathematical model. Otherwise, replacement of the analog hardware may necessitate a recalibration of the sensor and a remapping of the electromagnetic field in the motion box of the electromagnetic tracker.
U.S. Pat. No. 6,534,982 issued to Jakab, entitled “Magnetic resonance scanner with electromagnetic position and orientation tracking device” is directed to a system and method for determining the position and orientation of an object in a magnetic resonance scanner. In this reference, the magnetic resonance scanner is used in conjunction with an electromagnetic position and orientation tracking device to calculate the position and orientation of an object within the magnetic resonance scanner. The electromagnetic tracker may be used to track the object and a target, relative to a reference coordinate frame. Magnetic fields are used for location tracking of the object.
The magnetic field is generated by applying an electric current to a conductive wire loop. The generated magnetic field is sensed by a magnetic field sensor attached to the object. In the magnetic field sensor, an electric signal is measured which is proportional to the magnetic field. The electric signal is used to calculate the magnetic field by a magnetic field sensing model. For a solenoid type coil sensor, the magnetic field sensing model used is Faraday's Law of Induction. Models for other types of magnetic field sensors can be used, where the magnetic field sensing model is derived based upon the physical laws that dictate the operation of that sensor.
Using a model of the generated magnetic field, at an estimated location in the reference coordinate frame, an estimated magnetic field can be calculated. The magnetic field generation model includes a line segment approximation of the shape of the magnetic field source and additional line segments accounting for components that distort the magnetic field. The model compensates for distortions in the estimated magnetic field at the sensor, by accounting for the magnetic field generated by the gradient coils in the magnetic resonance scanner and for the magnetic fields generated by any surrounding conductive and ferromagnetic materials. The location and direction of the line segments is determined through test measurements of the currents in the gradient coils and the resulting magnetic fields. The Biot-Savart Law is used in the magnetic field generation model to calculate the magnetic field generated by each line segment.
Comparing the estimated magnetic field to the measured magnetic field, an error is computed. The estimated location of the magnetic field sensor is then changed and the steps repeated until the error between the estimated magnetic field and the measured magnetic field falls below an acceptable level. When the error falls below the acceptable level, the location of the sensor, based on its estimated location, provides a representation of the actual location of the sensor.
U.S. Pat. No. 6,400,139 issued to Khalfin et al., entitled “Methods and apparatus for electromagnetic position and orientation tracking with distortion compensation” is directed to distortion compensation in electromagnetic position and orientation tracking systems. The method of Khalfin is applicable specifically to electromagnetic tracking systems where the components of an AC electromagnetic field are sensed within a bounded volume. One or more probe sensors are placed on an object being tracked within the bounded volume. Each probe measures the magnetic induction vector components of the field generated by an electromagnetic radiation source, to determine the position, orientation and movement of the object within the bounded volume. To compensate for electromagnetic distortion, the method of Khalfin employs at least one stationary sensor, termed a witness sensor, in addition to the probe sensor(s) disposed on the object. Each witness sensor is supported at a known, fixed position and orientation relative to a reference frame of interest, at a point near or within the bounded volume, and close to the sensors on the object being tracked.
The measurements of each probe sensor, and each witness sensor, are provided to a processing unit operative to compute the position and orientation of the object in the presence of electromagnetic field distortions. The processor uses data from each witness sensor to compute parameters, such as the position, orientation and strength, of an effective electromagnetic source or sources. The effective source(s), which may be treated for the sake of simplicity as a point source or a dipole, produce the same electromagnetic field as a superposition of the real electromagnetic field, in addition to the field distortion in the proximity of the witness sensors. In the case of a non-distorted environment, the effective source(s) will be identical to the real source. This equality allows the computed parameters of the effective source(s) to be used as inputs to the computation of the position and orientation, as measured by each probe sensor, as if the object is in the non-distorted electromagnetic field produced by the effective source(s).
U.S. Pat. No. 6,201,987 issued to Dumoulin, entitled “Error compensation for device tracking systems employing electromagnetic fields” is directed to a system for the real-time localization of a device using electromagnetic (or Radio Frequency) fields, which compensates for eddy currents in the presence of eddy current inducing structures. The system is augmented with hardware and/or software which compensates for the eddy currents and is capable of more accurately tracking an invasive device in the body of a subject. In the system of Dumoulin, current patterns applied to transmit coils of the tracking system are modified to compensate for the effect of the eddy currents. Eddy currents are compensated for by modifying the waveforms which are applied to the transmitter coils. Ideally, in the absence of eddy currents, the current and the resulting applied field are uniform in time. In the presence of electrically conducting structures, however, eddy currents are created, which oppose the field created by the transmit coil. These currents, and the field created by them, decay over time.
The modified waveform of the current supplied to the coils is a linear combination of the current needed to create the desired electromagnetic field in the region of interest, and one or more error terms. The error terms are determined experimentally during system calibration and are mathematically modeled as a series of exponential functions having a given amplitude and time constant. The error terms, in the current applied to the transmit coils, effectively cancel the magnetic fields created by eddy currents within the tracking region and result in an actual electromagnetic field which is close to the desired ideal electromagnetic field. The fidelity of the electromagnetic field is further increased by reducing eddy currents within the eddy current inducing structures. This is done by constructing shield coils which are placed between the transmit coil and the eddy current inducing structures. Magnetic fields are created by these shield coils which cancel the magnetic fields created within the eddy current inducing structures without substantially altering the electromagnetic fields in the region over which the invasive device is tracked.
U.S. Pat. No. 5,646,525 issued to Gilboa, entitled “Three dimensional tracking system employing a rotating field” is directed to a system for determining the position and orientation of a helmet worn by a crew member in a vehicle. The vehicle includes a generator, which produces a rotating magnetic and electric dipole field of fixed strength, orientation and frequency within a portion of the vehicle. The system includes a plurality of sensors which each generate a signal proportional to the electric or magnetic fields detected at a point associated with the helmet. The sensors each comprise three orthogonal coils or three orthogonal detectors, such as Hall detectors. The system also includes calculation circuitry responsive to the generated signal for ascertaining the coordinates of the point with respect to the generator, using a simple mathematical model, and for determining the position and orientation of the helmet.
The generated signal is proportional to the time derivative of the magnetic field flux along the axis of a particular coil, or detector, in the sensor. Since the generator produces a rotating magnetic dipole field, the magnetic field strength, and hence the magnitude of the generated signal, at a specific point in space, will oscillate in time between a maximum and a minimum value. According to the simple mathematical model, the maximum and minimum values will depend only on the elevation angle of the point from the generator and the distance of the point from the generator. By determining the maximum and minimum of the square of the electric and magnetic field strengths, the calculation circuitry can determine the position and orientation of the helmet.